A quick draft of a monster add-on...
After you've made a monster, if it represents a truly horrible terror (a true abomination, horror from another realm, or extremely disgusting undead, for example) or else inspires supernatural awe and fear, answer the following:
Is it huge? If so, hold 1.
Is it solitary? If so, hold 1.
Is it terrifying? If so, hold 1.
Is it not of this reality? If so, hold 1.
Does it have a special attack or quality that inflicts horrific mental wounds? If so, note the attack and hold 1.
By default, anyone who witnesses the monster in all it's horrible (or awe-inspiring) glory takes 1d4 sanity damage. For each you've held, increase the damage size by 1, to a max of d12. If the creature has a mental attack, as above, they can also inflict their sanity damage through that attack.